The present invention relates to a process for producing images on a photosensitive material comprising a support and a normally positive-working photosensitive layer, in which the photosensitive layer is activated by exposure to ultraviolet light, imaged by means of thermal energy and developed with an aqueous-alkaline developer.
The advance in digital technique and the increasing supply of appropriate storage media has also produced a rise in the demand for printing plates which can be imaged by digital procedures. In the art of platemaking, various proposals have already been made which are directed toward imaging printing plates without the intermediate step of using a silver film mask.
German Offenlegungsschrift 23 18 133 (corresponding to U.S. Pat. No. 3,836,709) describes a process for the production of a polymeric image by means of laser beams. In this process, a layer having a photohardenable composition is required.
German Auslegeschrift 22 31 815 discloses the imaging of non-photosensitive layers by means of electron beams to produce negative reliefs. The process is technically expensive and due to the type of coating used it is only suitable for producing relatively small print runs. Owing to the technical expenditure and the small print runs obtained, the process has not become accepted in practice.
In U.S. Pat. No. 3,664,737, a printing plate carrying a photosensitive, negative-working layer is described, which is irradiated with a laser, while German Offenlegungsschrift 24 48 325 (corresponding to U.S. Pat. No. 4,034,183) discloses a process for the manufacture of planographic printing forms, in which a support coated with a hydrophilic non-photosensitive layer is irradiated imagewise by means of laser beams and the irradiated material is used for printing in a planographic printing machine, without any further treatment steps.
German Patent 27 28 947 (corresponding to British Patent 1,583,329) describes a process for preparing planographic printing forms, in which a planographic printing plate, which is formed of an anodically oxidized aluminum support and a photosensitive layer comprising a negative-working, photohardenable diazonium compound and an amino resin is irradiated imagewise with a laser beam.
European Patent Application 0 164 128 describes photosensitive copying materials and a process for imaging these materials by means of a YAG laser.
The methods which have become known in the art are, however, attended by various disadvantages. It is either necessary to use inefficient lasers for the imaging procedure, for example, argon-ion lasers operating in the ultraviolet spectral region, or the systems used, for example CO.sub.2 lasers, require great expenditure in view of safe practice and equipment technique.
Previously known layers which are sensitive to laser radiation also have the disadvantage that, on the one hand, the relatively highly photosensitive layers, such as those described in German Offenlegungsschrift 27 28 858 (corresponding to British Patent 1,583,330) for exposure to an argon-ion laser, have only a very limited thermal stability and consequently an insufficient shelf-life while, on the other hand, the layers according to German Offenlegungsschrift 24 48 325 (corresponding to U.S. Pat. No. 4,034,183) may have a very high thermal stability, but require extremely high energies for exposure. It is another disadvantage that reduction of the energies required for hardening the layer is invariably at the expense of thermal stability.
U.S. Pat. No. 4,544,627 discloses a negative-working imaging process, wherein the photosensitive material containing in its photosensitive layer an o-quinone diazide compound and a basic compound which facilitates conversion of the quinone diazide compound into an insoluble indene carboxylic acid derivative, is subjected to an overall irradiation, then imagewise exposed to laser light and subsequently developed. It has been found, however, that layers of that kind are not yet up to the highest standards, since their sensitivity to light is insufficient and they are not fully adapted for work in step-and-repeat machines.